Not long in the past, nearly all documents created had only a single author. Books, articles, scholarly papers, and so on were almost invariably the work of a single individual. Collaboration on a given piece of text was rare, no doubt in large part because it was difficult—involving the physical transfer of pieces of paper from one co-author to another. For example, we think of the Christian Bible as a group-authored document, but in fact it was compiled from numerous sources long after they had been completed. Marie and Pierre Curie managed collaborative authorship, but they were a married couple. Not all authors wishing to collaborate have that advantage.
The advent of electronic documents greatly simplified the process of collaborative authorship, because it allowed the moving of bits to replace the moving of physical paper. Co-authorship, or tightly coupled author-editor exchanges, became more common. This has been particularly beneficial in technical documents, where each author may have a slightly different area of expertise.
But until recently, co-authorship involved either a timesharing model, in which the authors alternated the (virtual) possession of the document, or a parallel-access-and-merge model, in which authors had possession simultaneously, but periodically had to merge changes into a single version to prevent their individual versions from diverging too far. The recent advent of simultaneous editing capabilities in cloud-computing services changed that, and allowed for a much more interactive level of collaborative authorship wherein multiple authors can edit a single document at the same time.